U.S. patent number 4,347,144 [Application Number 06/318,860] was granted by the patent office on 1982-08-31 for process for the purification of effluent.
This patent grant is currently assigned to Hoechst Aktiengesellschaft. Invention is credited to Kurt Bodenbenner, Helmut Perkow, Helmut Vollmuller.
United States Patent |
4,347,144 |
Bodenbenner , et
al. |
August 31, 1982 |
Process for the purification of effluent
Abstract
Effluent containing organic impurities which are difficult to
degrade is purified by oxidation in the aqueous phase at
temperatures of 100.degree. to 310.degree. C. and under elevated
pressure. This is effected by initially treating the effluent in a
first stage with gases containing oxygen, but without the addition
of catalysts, until the C.O.D. value has been reduced by 50 to 98%.
The effluent which has received preliminary treatment in this way
is subsequently treated, in a second stage, with an oxidizing agent
stronger than oxygen, in the liquid phase and at the same, or at a
higher, temperature, until the C.O.D. value has fallen to
approximately 0 g/l.
Inventors: |
Bodenbenner; Kurt (Wiesbaden,
DE), Perkow; Helmut (Hofheim am Taunus,
DE), Vollmuller; Helmut (Kelkheim, DE) |
Assignee: |
Hoechst Aktiengesellschaft
(Frankfurt am Main, DE)
|
Family
ID: |
6116296 |
Appl.
No.: |
06/318,860 |
Filed: |
November 6, 1981 |
Foreign Application Priority Data
Current U.S.
Class: |
210/761 |
Current CPC
Class: |
C02F
11/08 (20130101); C02F 1/72 (20130101) |
Current International
Class: |
C02F
1/72 (20060101); C02F 11/08 (20060101); C02F
11/06 (20060101); C02F 001/72 () |
Field of
Search: |
;210/758,759,760,761,762 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Therkorn; Ernest G.
Attorney, Agent or Firm: Berman, Aisenberg & Platt
Claims
We claim:
1. A process for the purification of effluent containing organic
impurities which are difficult to degrade, by oxidation in an
aqueous phase at temperatures of 100.degree. to 310.degree. C. and
under elevated pressure, the effluent being initially treated in a
first stage with gases containing oxygen, but without addition of
catalysts, until the C.O.D. value has been reduced by 50 to 98%,
which comprises subjecting the effluent which has received
preliminary treatment in this way, subsequently, in a second stage,
to treatment with an oxidizing agent stronger than oxygen, in the
liquid phase and at the same, or at a higher, temperature, until
the C.O.D. value has fallen to approximately 0 g/l.
2. A process as claimed in claim 1, wherein nitric acid or a
nitrate are used as the oxidizing agent in the second stage.
3. A process as claimed in claim 2, wherein 0.02 to 0.05 mole of
nitric acid or nitrates per g of C.O.D. in the effluent leaving the
first stage, are employed for oxidation in the second stage.
4. A process as claimed in claim 1, wherein the effluent containing
the organic impurities which are difficult to degrade, originates
from the manufacture of secondary products of cellulose.
5. A process as claimed in claim 1, wherein the effluent employed
also contains chlorides in addition to the organic impurities.
Description
The invention relates to a process for the purification of effluent
containing organic impurities, by a two-stage oxidation.
It is known that water containing organic impurities can be
purified by oxidation with gases containing oxygen at temperatures
between 235.degree. and 370.degree. C. and under a pressure which
is sufficient to keep at least a part of the water in the liquid
phase. The "Zimmermann Process", as it is called, is described, for
example, in U.S. Pat. Nos. 2,665,249 and 2,824,058.
Some effluents contain organic impurities which require
temperatures around or above 320.degree. C., and thus total
pressures of up to 250 bars, for their complete degradation by wet
oxidation with air. This makes the equipment large and expensive,
particularly if it is intended to deal with chloride-containing
effluents, which are extremely corrosive at high temperatures and
therefore require expensive materials, such as .RTM. Hastelloy,
titanium or tantalum. In addition, as the total pressure increases,
the cost of compressing the air employed becomes increasingly
higher.
It is possible to improve the mass transfer of the oxygen into the
aqueous phase, for example by installing stirrers in every stage of
a cascade reactor (German Offenlegungsschrift No. 2,435,391). The
increase in the reaction rate thus achieved makes it possible, in
the case of some effluents, to reduce somewhat the reaction
temperatures required. However, the process is expensive from a
technical point of view, since an expensive, chambered reaction
apparatus containing several shaft seals against high pressure is
required.
It is also possible, by using a catalyst, to reduce the temperature
required and thus the pressure for a constant degradation
throughput. The corrosion problems and thus the equipment costs can
be reduced considerably in this way (German Offenlegungsschrift No.
2,535,485). The catalysts customarily used are heavy metal
compounds, in particular copper salts. However, owing to the
fungicidal, algicidal and bactericidal action of the copper ions or
the toxic action of other heavy metal ions, complete removal of the
catalyst after the oxidation is absolutely necessary. Heterogeneous
catalysts on a supporting material tend to become inactivated
through contamination with substances contained in the effluent or
become inactive as a result of catalyst poisons.
It is also known that wet oxidation with oxygen leads in many cases
only to a partial degradation of the organic impurities (U.S. Pat.
No. 3,977,966, Example 1; German Offenlegungsschrift No.
2,445,391). In these cases, it has been considered sufficient to
subject the effluent thus pre-treated to biological treatment and
then to put it into the main outfall (compare Chem.Ing. Technik 52
(1980) No. 8, page A440). However, it is a disadvantage in this
procedure that some compounds are only very slowly degraded
biologically and the quantity of them in the biological sewage
treatment plant is therefore hardly reduced.
It is also possible to employ nitric acid or salts thereof for the
oxidation (German Offenlegungsschrift No. 2,262,754). However, in
this case it is necessary to employ at least the stoichiometric
quantity of HNO.sub.3 or a considerable excess, which--particularly
at high C.O.D. values--makes the process uneconomical. It is
indicated in German Offenlegungsschrift No. 2,748,638 that, if a
less than stoichiometric quantity of HNO.sub.3 is employed, the
fractions of organic compounds which are destroyed are, in the
main, only those which are biologically toxic and difficult to
degrade. Thereby, it is alleged, effluents which are difficult to
degrade by biological means are rendered accessible to purification
in a biological sewage treatment plant. It is a disadvantage in
this that the biological purification causes further expense.
Furthermore, however, the C.O.D. value in the sewage treatment
plant is only slightly reduced, insofar as it is due to impurities
which are only slowly degraded by biological means, that is to say
are not measured by the BOD.sub.5 value.
The problem therefore existed of finding a process which manages
without a biological sewage treatment plant, possesses the economy
of the wet oxidation process using oxygen, but, nevertheless, makes
it possible to purify the effluent virtually completely. A process
has now been found for purifying effluent containing impurities
which are difficult to degrade, by oxidation in an aqueous phase at
temperatures of 100.degree. to 310.degree. C. under elevated
pressure, the effluent being initially treated in a first stage
with gases containing oxygen, but without the addition of
catalysts, until the C.O.D. value has been reduced by 50 to 98%.
The process comprises subjecting the effluent which has received
preliminary purification in the first stage, subsequently, in a
second stage, to treatment with an oxidizing agent stronger than
oxygen, in the liquid phase and at the same, or at a higher,
temperature, until the C.O.D. value has fallen to about 0 g of
O.sub.2 per l. The C.O.D. value of the effluent is determined using
dichromate in the presence of silver salts. This method is
described in detail in "Deutsche Einheitsverfahren fur Wasser-,
Abwasser- und Schlammuntersuchung" (German Standard Processes for
the Examination of Water, Effluents and Sludge), 3rd edition,
1971.
Oxidizing agents which are ranked as stronger than oxygen are
substances which, in a 1-molar solution, or under a pressure of
0.98 bar, in water at the pH value of the solution to be purified,
have a higher redox potential than O.sub.2, such as, for example,
ozone or chlorate. Oxidizing agents which are preferred in the
second stage are nitric acid or nitrates. In this case, pH values
of 1 to 3 in the oxidation are preferred; however, oxidation can
also be carried out, even if more slowly, at higher pH values. The
quantity of nitric acid or nitrate is about 0.02 to 0.05 mole/g of
C.O.D, in particular 0.025 to 0.03 mole/g of C.O.D.
Under the reaction conditions, nitric acid and nitrates exhibit
hardly any tendency to form oxides of nitrogen, but are reduced
almost completely to give nitrogen.
Since no endeavour is made, in the process according to the
invention, to degrade organic impurities completely in the first
stage, it is possible to operate this stage at temperatures which
are 10.degree. to 80.degree. C., preferably 40.degree. to
60.degree. C., lower than the temperatures required for complete
degradation. The "temperature required for complete degradation" is
understood in this context to mean the temperature at which the
C.O.D. value of an effluent is decreased by 95 to 100%, if the
reaction is carried out in a stirred autoclave, with a residence
time of 30 minutes and with a 10 to 20% excess of oxygen. In the
process according to the invention, the effluent does not have to
be heated to such a high temperature, the equipment can be designed
for lower pressures and the corrosive attack, in particular by
chloride ions, which frequently occur in effluents, is reduced.
In the first stage, the total pressure should be 1.0 to 3.5 times,
in particular 1.5 to 2.5 times, the partial pressure of water vapor
at the operating temperature.
The effluent leaving the first stage is fed to the second stage
without cooling. In the second stage an increase in temperature can
take place, caused by a newly commencing degradation of the organic
compounds when the oxidizing agent is added. In general, therefore,
the second stage also takes place at temperatures of 100.degree. to
310.degree. C. Temperatures of 200.degree. to 300.degree. C. are
preferred in both stages. The process can be carried out at lower
temperatures, in particular when there is a high content of very
readily oxidizable organic substances, such as, for example,
formaldehyde. If, as well as readily degradable substances, organic
impurities which are difficult to degrade are also present in the
effluent, account can be taken of this in the second stage by
selecting the temperature and the residence time accordingly.
The process according to the invention is particularly suitable for
effluents having a C.O.D. of at least 10 g/l, in particular at
least 20 g of C.O.D./l. Effluents which are free from heavy metal
ions are preferred, since a further treatment stage is then no
longer required after the oxidation. The process according to the
invention is suitable for purifying many types of effluent, in
particular for effluents originating from the manufacture of
secondary products of cellulose. It is thus possible, in most
cases, to purify the effluent employed to such an extent that the
C.O.D. value has fallen to less than 2% of its original value and a
further (for example biological) after-treatment is
superfluous.
One possible means of putting into practice, on an industrial
scale, the process according to the invention is illustrated by
means of the flow-sheet of the FIGURE:
Effluent containing organic impurities is compressed by means of a
high pressure pump (1) to the operating pressure of the reactor 5
and is warmed, via line 12, in a counter-current heat exchanger 4,
together with a partial stream, fed via (2), of the air compressed
in compressor 3. The pre-heated effluent is then fed to a cascade
reactor 5 having sieve trays (8, 8A and 8B), from below through the
feed line 6. At the same time, the remainder of the air compressed
by pump 3 is fed into the reactor through the feed line 7. The
reaction mixture is fed to a separator 9 via the line 17. In this
separator the inert constituents of the air and the reaction gases
are separated from the liquid phase. The effluent, which is
partially freed from its organic constituents is now fed to a
second reactor 10 via (18). (In principle, reactor 5 could also be
connected to reactor 10, for example could be placed on top of the
latter. The gas phase formed in 5 would then only be separated from
the liquid phase together with the gas phase from 10--this is not
drawn).
Nitric acid is then added in the reactor 10, via a pump 11 and line
19, in a quantity (calculated as C.O.D; 40 g of C.O.D. correspond
to 1 mole of HNO.sub.3) which is stoichiometric in relation to the
residual organic constituents. Disregarding an increase in
temperature caused by the oxidation with nitric acid, reactor 10 is
operated at the same temperature as the reactor 5. The pressure in
10 is lower than the pressure in 5, by the partial pressure of air
discharged at the head of 9.
Reactor 10 is preferably constructed, like reactor 5, as a bubble
column having sieve trays arranged in cascade. Another type of
reactor, such as, for example, a stirred vessel or a flooded packed
column, is also possible, however. Any materials which are
customary in the wet oxidation process with air are suitable as
materials of construction; for example titanium alloyed with
palladium can be employed for an effluent containing chloride.
The purified effluent is then fed, together with the reaction
gases, via line 20 to a heat exchanger 4 and is there cooled in
counter-current with the fresh effluent.
Excess heat can be utilized for the production of steam in a
vaporizer (not drawn). From 4 the mixture passes via line 24 to the
separator 13. The reaction gases and the effluent are separated
here. The reaction gases, which can be contaminated with NO.sub.x,
for example when starting up the apparatus, are taken off at the
head of 13 and pass via 21 to the washer 14. Here they are purified
in a part stream of the fresh effluent which is fed in via 15. The
loaded stream of wash water leaves 14 at its base and is recycled
to the effluent to be purified via line 22, in which pump 16 is
inserted. Emission of NO.sub.x is thus prevented with certainty and
the nitric acid is utilized entirely for the oxidation. Purified
effluent is withdrawn via line 23 at the base of 13. The invention
is illustrated in greater detail by means of the following
examples.
EXAMPLE 1
The effluent from manufacture of secondary products of cellulose
has a C.O.D. value of 40 g/l. Using wet oxidation in a shaking
autoclave and a quantity of air 50% greater than the
stoichiometrical requirement, a temperature of 330.degree. C. and a
pressure of 220 bars are required, at a residence time of 1 hour,
for 95% reduction in C.O.D.
Using wet oxidation alone, with nitric acid as the oxidizing agent,
complete reduction of C.O.D. is achieved at 250.degree. C. However,
90 kg of 65% strength HNO.sub.3 are required per m.sup.3 of
effluent.
If the same effluent is subjected to the process according to the
invention, the C.O.D. value can initially be reduced by 75% to 10
g/l in the first stage by wet aerial oxidation at 280.degree. C.
and 110 bars. For complete oxidation in the second stage it is only
necessary to add 22 kg of 65% strength HNO.sub.3 per m.sup.3 of
effluent. Here too, the residence time in the first stage is one
hour.
EXAMPLE 2
Another effluent, which also originates from the manufacture of
secondary products of cellulose (C.O.D. value 25 g/l), is extremely
corrosive at the temperature of 310.degree. C. (and a pressure of
170 bars) which is required for degradation by wet oxidation,
because it contains 8% by weight of sodium chloride. If the 2-stage
process according to the invention is used, a C.O.D. of 5 g/l (80%
reduction) is achieved by wet aerial oxidation at a temperature as
low as 270.degree. C. and a pressure as low as 93 bars (residence
time 1 hour). The second stage, similarly at 270.degree. C.,
requires 11 kg of 65% strength HNO.sub.3 as the oxidizing agent per
m.sup.3 of effluent. As a result of reducing the reaction
temperature by 40.degree. C., the corrosive attack of the effluent
is reduced, so that the oxidation can be carried out without
problems in titanium equipment (preferably alloyed with
palladium).
* * * * *